Ageing tantalum solid electrolyte capacitors is a big industrial problem, because this step, which is necessary for the production of high performance capacitors to which users have gradually become accustomed, accounts for an appreciable proportion of the production cost. Despite the accelerated ageing processes of current practice nowadays, this step is still lengthy and a duration of about twenty hours is usually considered necessary and this involves manipulations which give rise to heavy expenditure both from the viewpoint of labour and from the viewpoint of the necessary capital outlay. It is known to carry out an accelerated ageing by maintaining the capacitor under a direct voltage, while bringing it to a temperature close to or above the maximum temperature at which it is to operate. An example of such process is described in U.S. Pat. No. 3,495,311 assigned to International Standard Electric Corporation issued on Feb. 2, 1970, which describes an accelerated ageing under voltage, performed during the tinning step. U.S. Pat. No. 3,653,119, assigned to Sprague and filed on Dec. 28, 1967, describes an accelerated ageing under voltage which is to take place after the anode formation by immersion in a fused salt bath, the melting temperature of which is between 250.degree. C. and 400.degree. C. The applied voltage is about 0.15 times the anodizing voltage, which remains below 0.5 times the rated voltage. One advantage of the latter process resides in that it is a batch process, and it allows automatic protection of the anodes in the course of the treatment even if one of them proves to be defective (even shortcircuited) as a result of the formation of a gaseous envelope around the defective part, which electrically insulates it from the molten bath. However such an ageing process is so complex that it is of very little use in production.
At present, it is usual to proceed with ageing of the anodes as follows: the anodes, after formation, are mounted on holders, in series with individual protective resistors, and then subjected to the following treatment:
4 hours at ambient temperature at 1.4 times the rated voltage with a series resistance of 10 kilohms
15 hours at 85.degree. C. at rated voltage with a series resistance of 1 kilohm, that is to say, a total treatment duration of 19 hours, preceded by mounting of the anodes, taken one by one, on a carrier bar and changing over the protective resistors between the two operating stages. It is readily apparent from the foregoing brief description of the process that the ageing operation is costly both in capital outlay and in labour.
The present invention has essentially for its object to provide a process for accelerated ageing under voltage which utilizes the anodes mounted on the carrier bar used for the oxidizing stage, without requiring any particular manipulation in the ageing stage, the duration of which is reduced by a factor of about 10 in relation to the previously described process and which affords automatic protection of the neighbouring capacitors in the event of a defective anode without the need of any limiting resistor. It further has the advantage that it permits, in the course of the ageing, a self-healing of certain defective capacitors by reoxidation of the tantalum at the weak points.